International envoy Kofi Annan has given the Syrian government a 48 hour deadline to cease all violence against its citizens and to withdraw its heavily armed troops from local cities and towns.

The announcement comes as the original United Nations-backed peace plan deadline set for today has expired.

Annan said the situation was not as he had hoped. He wrote in a letter to the U.N. Security Council that the Syrian government must "seize the opportunity to make a fundamental change of course" by 6 a.m. Thursday.

More than 100 people have been executed by Syrian security forces since late last year. A new report from the group Human Rights Watch has documented at least 85 mass slayings of civilians, including women and children, and the capture or wounding of 16 others.

An official from the group warns that these numbers may be only the tip of the iceberg.

The report went on to site similar atrocities committed by Syrian opposition forces against government troops. Civilian resistance members however have indicated to Annan that they would observe a ceasefire once "Syrian forces withdraw from cities."